Parasitic worms, also known as helminths,
are large
macroparasites; adults can generally be seen with the naked eye. Many are
intestinal worms that are
soil-transmitted and
infect the
gastrointestinal tract. Other parasitic worms such as
schistosomes reside in blood vessels.
Some parasitic worms, including
leeches
Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented b ...
and
monogenea
Monogeneans are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they have both male and female repr ...
ns, are
ectoparasites thus, they are not classified as helminths, which are
endoparasites
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
.
Parasitic worms live in and feed in living
hosts
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
People
*Jim Host (born 1937), American businessman
*Michel Host ...
. They receive
nourishment and protection while disrupting their hosts' ability to absorb
nutrients. This can cause weakness and
disease
A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
in the host, and poses a global health and economic problem. Parasitic worms cannot reproduce entirely within their host's body; they have a life cycle that includes some stages that need to take place outside of the host.
Helminths are able to survive in their mammalian hosts for many years due to their ability to manipulate the host's
immune response
An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which coul ...
by secreting immunomodulatory products.
[Jirillo, E., Magrone, T., Miragliotta, G. (2014)]
"Immunomodulation by Parasitic Helminths and its Therapeutic Exploitation"
In: Pineda, M.A., Harnett, W. (eds.), ''Immune Response to Parasitic Infections'' (Vol. 2, pp. 175–212), Bentham eBooks, , . All parasitic worms produce
eggs during reproduction. These eggs have a strong shell that protects them against a range of environmental conditions. The eggs can therefore survive in the environment for many months or years.
Many of the worms referred to as helminths are intestinal parasites. An infection by a helminth is known as
helminthiasis
Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths. There are numerous species of these parasites, which are ...
, helminth infection, or intestinal worm infection. There is a naming convention which applies to all helminths: the ending "-asis" (or in veterinary science: "-osis") is added at the end of the name of the worm to denote the infection with that particular worm. For example, ''
Ascaris
''Ascaris'' is a genus of parasitic nematode worms known as the "small intestinal roundworms", which is a type of parasitic worm. One species, '' Ascaris lumbricoides'', affects humans and causes the disease ascariasis. Another species, ''Ascar ...
'' is the name of a type of helminth, and
ascariasis is the name of the infection caused by that helminth.
Taxonomy
Helminths are a group of organisms which share a similar form but are not necessarily related as part of
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
. The term "helminth" is an artificial term. There is no real consensus on the taxonomy (or groupings) of the helminths, particularly within the
nematodes.
The term "helminth" contains a number of
phyla Phyla, the plural of ''phylum'', may refer to:
* Phylum, a biological taxon between Kingdom and Class
* by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another
Phy ...
, many of which are completely unrelated. However, for practical considerations the term is currently used to describe four phyla with superficial similarities:
Annelida (ringed or segmented worms),
Platyhelminthes
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegm ...
(
flatworms),
Nematoda (roundworms), and
Acanthocephala (thorny-headed worms).
The phylum
Platyhelminthes
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegm ...
includes two classes of worms of particular medical significance: the
cestodes (tapeworms) and the
trematodes (flukes and
blood flukes
''Schistosoma'' is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed ''schistosomiasis'', which is considered by the World Health Organi ...
), depending on whether or not they have segmented bodies.
There may be as many as 300,000 species of parasites affecting vertebrates, and as many as 300 affecting humans alone.
Helminths of importance in the
sanitation
Sanitation refers to public health conditions related to clean drinking water and treatment and disposal of human excreta and sewage. Preventing human contact with feces is part of sanitation, as is hand washing with soap. Sanitation syste ...
field are the human parasites, and are classified as
Nemathelminthes (nematodes) and
Platyhelminthes
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegm ...
, depending on whether they possess a round or flattened body, respectively.
Ringworm (dermatophytosis) is actually caused by various
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
and not by a parasitic worm.
Reproduction and life cycle
The
lifetime of adult worms varies tremendously from one species to another but is generally in the range of 1 to 8 years (see following table). This lifetime of several years is a result of their ability to manipulate the immune response of their hosts by secreting immunomodulatory products.
Helminths can be either hermaphroditic (having the sex organs of both sexes), like tapeworms and flukes (not including the blood fluke), or have their sexes differentiated, like the roundworms.
All helminths produce eggs (also called ova) for reproduction.
Eggs
Generally, thousands or even hundreds of thousands of eggs are produced each time the female worm deposits its eggs - a process called
oviposition. There is a large variation in the number of eggs produced by different species of worm at one time; it varies in the range of 3,000 to 700,000. The frequency of egg deposition from an adult helminth is generally daily, and can occur up to six times per day for some ''Taenia'' species. Adult trematodes lay smaller numbers of eggs compared to cestodes or nematodes. However, the egg develops into a
miracidia from which thousands of ''
cercariae'', or swimming larvae, develop. This means that one egg may produce thousands of adult worms.
Helminth eggs remain viable for 1–2 months in crops and for many months in soil, fresh water, and
sewage, or even for several years in
feces,
fecal sludge (historically called
night soil), and
sewage sludge
Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term " septage" also refers to sludge from simple wastewater treatment but is connected to s ...
– a period that is much longer compared to other microorganisms.
[Feachem, R., Bradley, D., Garelick, H., Mara, D. (1983). Sanitation and Disease: Health Aspects of Excreta and Wastewater Management. John Wiley and Sons, New York, NY.]
Eggs can reach the soil when polluted
wastewater,
sewage sludge
Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term " septage" also refers to sludge from simple wastewater treatment but is connected to s ...
or
human waste are used as
fertilizer. Such soil is often characterized by moist and warm conditions. Therefore, the risk of using contaminated wastewater and sludge in agricultural fields is a real problem, especially in poor countries, where this practice is prevalent.
[ ] Helminth eggs are regarded as the main biological health risk when applying sewage sludge, fecal sludge or fecal matter on agricultural soils.
The eggs are the infective stage of the helminths’ life cycle for causing the disease
helminthiasis
Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths. There are numerous species of these parasites, which are ...
.
Helminth eggs are resistant to various environmental conditions due to the composition of the egg shell. Each helminth egg species has 3 to 4 layers with different physical and chemical characteristics:
Due to this strong shell, helminth eggs or ova remain viable in soil, fresh water and sewage for many months. In feces, fecal sludge and sewage sludge they can even remain viable for several years.
Helminth eggs of concern in
wastewater used for
irrigation have a size between 20 and 90 μm and a relative density of 1.06–1.23.
It is very difficult to inactivate helminth eggs, unless temperature is increased above 40 °C or moisture is reduced to less than 5%.
Eggs that are no longer viable do not produce any larvae. In the case of ''
Ascaris lumbricoides
''Ascaris lumbricoides'' is a large parasitic worm that causes ascariasis in humans. A roundworm of genus ''Ascaris'', it is the most common parasitic worm in humans. An estimated one-sixth of the human population is at some point infected by ...
'' (giant roundworm), which has been considered the most resistant and common helminth type, fertilized eggs deposited in soil are resistant to desiccation but are, at this stage of development, very sensitive to environmental temperatures: The reproduction of a fertilized egg within the eggshell develops at an environmental soil temperature about 25 °C which is lower than the body temperature of the host (i.e., 37 °C for humans).
However, development of the larvae in the egg stops at temperatures below 15.5 °C, and eggs cannot survive temperatures much above 38 °C. If the temperature is around 25 °C, the infectiousness occurs after nearly 10 days of incubation.
Larvae
Larvae hatch from eggs, either inside or outside the host, depending on the type of helminth. For eggs in moist soil at optimal temperature and oxygen levels, the embryo develops into an infective larva after 2 to 4 weeks, named "second-stage larva". Once ingested by a host, this larva has the ability to get out of the egg, hatch in the small intestine and migrate to different organs. These infective larvae (or "infective eggs") may remain viable in soil for two years or longer.
The process of larval maturation in the host can take from about two weeks up to four months, depending on the helminth species.
The following table shows the principal morphological and reproductive distinctions for three helminth groups:
Draft genomes for all categories of helminth have been sequenced in recent years and are available through the ParaSite sub-portal of
WormBase.
Use in medicine
Parasitic worms have been used as a medical treatment for various diseases, particularly those involving an overactive
immune response
An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which coul ...
.
As
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s have evolved with parasitic worms, proponents argue they are needed for a healthy immune system.
Scientists are looking for a connection between the prevention and control of parasitic worms and the increase in
allergies such as hay-fever in developed countries.
Removal of parasitic worms from areas is correlated with an increase in autoimmune disorders in those areas.
Parasitic worms may be able to damp down the immune system of their host, making it easier for them to live in the intestine without coming under attack.
This may be one mechanism for their proposed medicinal effect.
One study suggests a link between the rising rates of
metabolic syndrome in the developed worlds and the largely successful efforts of Westerners to eliminate intestinal parasites. The work suggests eosinophils (a type of white blood cell) in fat tissue play an important role in preventing insulin resistance by secreting interleukin 4, which in turn switches
macrophages into "alternative activation". Alternatively-activated macrophages are important to maintaining glucose homeostasis (i.e., blood sugar regulation). Helminth infection causes an increase in eosinophils. In the study, the authors fed rodents a high-fat diet to induce metabolic syndrome, and then injected them with helminths. Helminth infestation improved the rodents' metabolism.
The authors concluded:
Although sparse in blood of persons in developed countries, eosinophils are often elevated in individuals in rural developing countries where intestinal parasitism is prevalent and metabolic syndrome rare. We speculate that eosinophils may have evolved to optimize metabolic homeostasis during chronic infections by ubiquitous intestinal parasites….
Levels of infectiousness
Helminth eggs contained in wastewater, sewage sludge or human excreta are not always
infectious, i.e. able to cause the disease helminthiasis. Fertilized eggs and unfertilized eggs can exist side by side. Unfertilized eggs are identifiable under the microscope by their elongated shape. No larvae can hatch from these kinds of eggs. Therefore, unfertilized eggs do not pose a danger to human health.
Sanitation
Removal versus inactivation in wastewater and sludge treatment
In order to physically remove (but not inactivate) helminth eggs from wastewater, processes that remove particles, such as sedimentation, filtration or
coagulation-flocculation are employed. Therefore,
waste stabilization ponds (lagoons), storage basins,
constructed wetlands
A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development. ...
, rapid filtration or
upflow anaerobic sludge blanket (UASB) reactors can be used.
Helminth ova cannot be inactivated with chlorine, UV light or ozone (in the latter case at least not with economical doses because >36 mg/L ozone are needed with 1 hour contact time).
Inactivation of helminth ova can be achieved in
sewage sludge treatment
Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge treatment is focused on reducing sludge weight and volume to reduce transportation and disposal costs, and on red ...
where the temperature is increased over 40 °C or moisture is reduced to less than 5%.
Best results can be obtained when both of these conditions are combined for an extended period of time.
[Schmidt, G.D., Roberts, L.S. (1981). Foundations of Parasitology, second ed. C.V. Mosby Company, 795 pp] Details about the contact time under these conditions and other related environmental factors are generally not well-defined for every type of helminth egg species.
Helminth eggs are considered highly resistant biological structures.
Indicator organism
Helminth eggs (or ova) are a good
indicator organism to assess the safety of sanitation and reuse systems because they are the most environmentally resistant of all
pathogens (
viruses,
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
,
protozoa and helminths) and can in extreme cases survive for several years in soil.
Therefore, the presence or absence of viable helminth eggs ("viable" meaning that a larva would be able to hatch from the egg) in a sample of dried fecal matter,
compost or
fecal sludge is often used to assess the efficiency of diverse wastewater and sludge treatment processes in terms of pathogen removal. In particular, the number of viable
Ascaris
''Ascaris'' is a genus of parasitic nematode worms known as the "small intestinal roundworms", which is a type of parasitic worm. One species, '' Ascaris lumbricoides'', affects humans and causes the disease ascariasis. Another species, ''Ascar ...
eggs is often taken as an indicator for all helminth eggs in treatment processes as they are very common in many parts of the world and relatively easy to identify under the microscope. However, the exact inactivation characteristics may vary for different types of helminth eggs.
Diagnosis
Environmental samples
For the purpose of setting treatment standards and reuse legislation, it is important to be able to determine the amount of helminth eggs in an environmental sample with some accuracy. The detection of viable helminth eggs in samples of wastewater, sludge or fresh feces (as a diagnostic tool for the infection
helminthiasis
Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths. There are numerous species of these parasites, which are ...
) is not straight forward. In fact, many laboratories in developing countries lack the right equipment or skilled staff required to do so. An important step in the analytical methods is usually the concentration of the eggs in the sample, especially in the case of wastewater samples. A concentration step may not be required in samples of dried feces, e.g. samples collected from
urine-diverting dry toilet
A urine-diverting dry toilet (UDDT) is a type of dry toilet with urine diversion that can be used to provide safe, affordable sanitation in a variety of contexts worldwide. The separate collection of feces and urine without any flush water has ma ...
s.
Human stool samples
For medical purposes, the exact number of helminth eggs is less important and therefore most
diagnoses are made simply by identifying the appearance of the
worm
Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always).
Worms vary in size from microscopic to over in length for marine polychaete wor ...
or eggs in
feces. Due to the large quantity of eggs laid,
physicians can
diagnose
Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine "cause and effect". In systems enginee ...
using as few as one or two
fecal smears. The
Kato technique (also called the Kato-Katz technique) is a laboratory method for preparing human stool samples prior to searching for parasite eggs.
Eggs per gram is a laboratory test that determines the number of eggs per gram of feces in patients suspected of having a parasitological infection, such as schistosomiasis.
See also
*
Helminthology the study of parasitic worms and their effects on their
hosts
References
Further reading
*
Dickson Despommier
Dickson D. Despommier (June 5, 1940) is an emeritus professor of microbiology and Public Health at Columbia University. From 1971 to 2009, he conducted research on intracellular parasitism and taught courses on parasitic diseases, medical eco ...
, ''People, Parasites, and Plowshares: Learning from Our Body's Most Terrifying Invaders'',
Columbia University Press
Columbia University Press is a university press based in New York City, and affiliated with Columbia University. It is currently directed by Jennifer Crewe (2014–present) and publishes titles in the humanities and sciences, including the fie ...
, 2016 ().
External links
Parasitic Roundworm DiseasesWorld Health Organisation (WHO) topic page on Intestinal worms
{{DEFAULTSORT:Parasitic Worm
Ascaridida
Conditions diagnosed by stool test
Foodborne illnesses
Helminthiases
Intestinal infectious diseases
Tropical diseases
Parasitic protostomes
Sanitation